Fluctuating tachometer

Yep, did a search already. No luck here or the other place.

Riding along tonight for our maiden voyage. The tachometer fluctuates sporadically. Nothing specific seems to trigger it. Sometimes it just dips, sometimes it drops to zero. If anything, I think it does it more on deceleration than any other time. At one point it dropped to zero for about 10 seconds then picked back up. The bike runs exactly the same whether the tach is fluctuating or not.

I went through the gages and checked all connections on the rear of the unit and on the harness when I installed it. This is a used cluster I got off ebay because the lens was much clearer than the one I had. Everything else works great.

Any ideas?

 

Start checking individual wires for breaks or corrosion. If you check voltages I bet you will find a drop in voltage when the tach drops on a specific wire.

 

All the wiring and harnesses look good, and I'm not about to start tearing apart the looms. I'm sure there must be a simpler answer. I have no way to check voltage inside the tach, especially while riding. Intermittent problems like this are a pain to find since troubleshooting requires taking voltages at different points while the symptom is occurring. Looking at the wiring diagram I see the same wire that goes from the ignition box to the tach also signals the coil. Since it's not misfiring then I think the fault must be either at the tach itself or its connector.

I've heard of strange things happening at the tach on other bikes when the R/R goes out. Does the VF500 act the same way? Just something I'd like to rule out so I'm not chasing my tail.

 

I lost my tach when my R/R (Regulator/Rectifier) went out on my 86 VF500 recently - it is sensitive to low voltage
(I also lost my turn-signals, dash back-lights, and motor started running rough (not enough voltage for ignitors/coils)

The bad rectifier trashed my newer battery really quick

Check and clean the connectors and harness from the regulator to the main harness ... I also had an intermitant connection issue. The regulator is on the back side of the wiring panel just in front of the coolant over-flow bottle. (Pull the coolant over-flow to allow you get at the regulator)


These were my "healthy" used replacement R/R readings with a cold engine

Battery Voltage
-12.6V ignition off (Should not be below 12V)
-12.3V Igintion on
-11V starting (should not drop below 10.5 volt or bike may not start)
-14.8V at 5K rpm (should NOT exceed 15V at 5k rpm) recitifer is supposed to take alternator voltage to ground above 15V

Moose

 

Not sure about the 500's, my 750 the tach works from one of the CDI boxes. Check your connections there.

 

tachs work of of a diferential voltage system, the pulsed dc (its really AC) from your cdi or coil supply line is rectified by a small current diode. the result is a constant voltage that is placed across a inductor in the tach, tis is what causes the needle to lift. a constant voltage from the battery is placed across another inductor to pull the needle down, the differrence in voltages is what causes the needle to settle at some spot that is proportionate to the voltage of the ignition system. This voltage is also proportionate to the speed of the engine. NOTE: some "one wire" or "two wire" tachs use a magnet or various electronics to move the needle, though they are accurate for most purposes, they can be hard to solve for issues

Some tachometers pull the needle with faster engine speeds, others push it, the difference is on the loaction of the constant voltage inductor vs the input inductor.

that said, the cause could simply be a bad ground, but more often than not its the diode that stops working when hot. once cooled it starts up again. a basic test will not show this happening, it must be under a test load to heat it up. Hair dryers and the like will just kill it regardless. Some tachs have a power resistor across them to limit current going through them. this could be a place of concern as they are usualy wirewound ceramics and can be broken without showing on the outside.

The fact it keeps cutting out then comes back on leads me to look at the grounding, the 12v supply, and the incomming tach signal. a simple led and 300ohm resistor on the tach line will show you while riding if the tach signal is cutting out.
tach signal line - - - - - - - - - - - - - - - tachometer- - - gnd
\__300ohm__led__gnd
do not place led in line with tachometer.

intermitant ground can be proven by running a 14awg wire to the ground of the tach from the neg bat term. if all symptoms disapper then that is it. the same can be done for the positive supply to the tach, though, add a fuse and remove the original supply line from the tach, you do not want to double feed the tach or what it is connceted to.

these are my thoughts, and yes I expect someone to say im way off base. this is just some knowledge for you to consider.

zen

 

The 500's have the same setup then - the spark unit for the 1-3 coil also sends a signal to the tachometer.

The troubleshooting guide notes to check this circuit for continuity – from the yellow wire terminal at the tachometer end of the wire harness to the yellow wire terminal of the 1-3 coil.

If the circuit shows continuity (low resistance) the guide recommends - as the next step - to install a new spark unit. If the tach still doesn't work the next recommended step is to replace the tachometer.

Hopefully this is a simple connector fix and not an issue with the spark unit or the tach.

Let us know how you make out.

 

Is there any way to tell what type of electricity feeds the tach, i.e. DC Voltage, DC current, AC voltage, AC frequency, or some pulsed signal? I'd like to know what I should be looking for. Keep in mind that the same wire sends the signal to the coil to fire. I'm thinking it must be some form of pulse.

I checked some connections last night - R/R, and both connectors from the ignition units - only because it was convenient to do. No abnormalities found. I'll do some more riding tomorrow and see if anything has changed.

 

I once tried to find out the tacho error.

I had the power supply connected at instruments and hooked a signal generator between earth and the signal wire (can remember the color code but I take IC words it's the yellow one) and CDi unit - coils were isolated. Cannot recall the waveform I inputted there but it was either sine or square pulses, most likely square pulses as my gen produces those in high level above 6 Volts. Tacho worked fine while engine was off and then it was tested for accuracy.

Here is a summary:

Indication 1300 rpm was at 24Hz-actual 1440 rpm
Indication 3000 rpm was at 53Hz-actual 3180 rpm
Indication 6000 rpm was at 100Hz-actual 6000 rpm
Indication 12000 rpm was at 197Hz actual 11820 rpm

I believe if you have a small transformer of 9~12 Volts you can test the tacho (if no signal generator is available).
I would feed the signal voltage with a resistor a few hundreds ohms in series. @50 Hz you should get a reading around 3000 rpm. Then you can go on with fault tracing which most likely is wiring issue. I exclude the faulty CDi unit, the engine wouldn't work if there was a damage on it.

Hope it helps.

 

This is hugely informative! Awesome post! I don't have a signal generator but knowing it's a square wave helps a bunch. I can probably read it with a DVM on AC Hz and see if I can find the fault. I'm going for a little ride around the lake here shortly. We'll see if my jostling the connectors did anything.

 

Thanks for the comments, I am glad if I helped.

The output signal produced from the CDi box is actually a square wave with min voltage around 0.5 Volts (=VCE of the output transistor) and max voltage 12 Volts (Vbat). But when this is applied to the primary of the coil (which is in parallel with the tacho signal input) is deformed due to the highly inductive load. In any case the wave's shape won't be a problem as long as it reaches the tacho's voltage triggering level.
If you hook a DVM and measure the AC Hz the box produces you will find something as my previous post.

Depending of what you wanna test you can use:

for the signal carrying cable
1. Just ohm metering,
2. Signal AC Hz metering with the engine ON

For the tacho
You need to feed the tacho input an AC square wave signal around 9~12 Volts.

For the CDi
You need to feed the CDI input (pick-up coils) an AC sine wave signal of few hundreds of mV and monitor the output in AC Hz.

If you need more info just ask.

Oppss... I did a mistake due to European thinking. The above recommended transformer method will produce 60 Hz in USA and will give ~3600rpm

 

My initial fiddling did not help much. I completely tore the tach down, right down to cleaning the small IC board with isopropyl alcohol. All connections have been checked. I looked for a failing component on the board but didn't find anything. I went so far as to use a magnifying glass to look for loose or broken solder joints. Nothing. I don't think it's the wiring, BUT I decided to install each tachometer and both do the same thing. Unless there's a common mode failure with the tachometer, I'm left with the problem narrowed down to the harness.

I really hate tearing into the stock harness because it usually does more harm than good. I may continue to ride it until A) it fixes itself, B) leaves me stranded on the side of the road, or C) drives me completely insane. Since it's a Honda, I'm hoping for A, though C is much more likely.

 

If both tachs fluctuate then I'd be inclined to thinks it's the CDI box 1-3 which controls the tach. See if you can find/borrow one for testing. I had a 1-3 box fail on me. The tach would only reach 2500 then drop to zero. But it also killed those two cylinders. You said yours runs fine it's just the tack that's not right. I would suspect the box before wiring. Unless you have funky wiring on the bike from PO's.

 

If I were you before tearing the harness i would run an external cable from CDI 1-3 directly to the tacho (a cable in parallel with existing in harness Yellow cable). This would ommitt or verify the "yellow cable" factor.
I would try above first and if I had no luck then I would swap CDi s for testing.

If you post a photo of the PCB, traces and components side, I will have a look on it and I might come with suggestions.

 

I have seen the exact same on a friend's VF400F, but the drop of power was impossible to be unnoticed. Hawkrider said it's bike works fine, (I assume on the road test) so there shouldn't be any issue with the CDi box and in any case they are swappable.

 

The parallel wire to the yellow is a great idea! I'll try that.

It may be a few days before I get to it. I'm moving to VA from NY tomorrow.

 

Not necessarily. The 1-3 has the tach circuit in it, the 2-4 doesn't. Yes you can swap boxes around and the bike will run, but the tach won't work off the 2-4 box.

The tach circuit in the 1-3 box could be malfunctioning and not disturb the the firing of the coils. Hey it's electronics. I've seen crazier.

Just want to state what I said is for a 750. I'm not certain about a 500. But seeing it has two boxes I'm assuming they work the same. Yea, I know assume??

 

I attempted a repair on my friend's VF400F CDi unit, and as far as i can recall (it's more than 10 years ago) there was no special tach circuit in them. It seems the output to coil wire is also used as a tach pulse signal. The rest of circuitry is located in the tach instrument most likely to be an F to V converter.
CDis differ for sure as one carries the rev limiter while the second doesn't but I would dare to give 100% for the tach test they are totally swappable.

Totally agree with that!

 

Here's something I've had for awhile. This may help you in your troubleshooting. Don't remember where I got it so I can't acknowledge the person that wrote. It is good though.

**************************************************************


How to Diagnose and Repair V4 Honda Ignitors
Copyright 2000,2001,2002 Clifton Koch
Standard Disclaimer: This is a guide to help diagnose and repair Ignitor modules on Honda motorcycles. Follow these procedures at your own risk. While I've had good luck in disassembling and fixing these units, it's quite possible to permanently damage them if you're not careful.
I ask that you not copy any portion of this document for display or publication without my consent (note the copyright notice above). I ask this because of problems I've had in the past with mis-transcribed things I've written that have caused me a number of headaches. Feel free to make links to these pages. They are here to be seen.

Note: The pictures on this page can be clicked on to view a larger image.

So you think you've got a dead ignitor. First let's try to prove if it's an ignitor or some other problem. Ignitors with problems can often be intermittent, so you have to actually catch it failing to determine which one is at fault, but an intermittent problem where pairs of cylinders seem to suddenly drop out makes the ignitors your chief suspect.

Have a look at the above diagram. The diagram shows all of the connections that make up the ignition system on Honda V4 Motorcycles. Most of the V4 engined Hondas use a separate ignitor for each cylinder pair. If an ignitor is the source of the problem, two cylinders will be affected while the other two are not. If your problem affects more than two cylinders, ignitors are probably not at fault unless you're unlucky enough to have two bad ones. If you're convinced the problem is the ignition, but it affects more than two cylinders, check the battery, ignition switch, kill switch, main fuse, and wiring harness grounds for poor connections.

(Repair hint #1: when checking the main fuse, actually take it out and hold it in your hand. Main fuses have a nasty habit of cracking in ways that are not easily visible, but will fall apart into pieces when you actually take them out.)

(Repair hint #2: there's a ground loop from the harness that usually connects to the coil mounting brackets that can cause odd problems if not connected.)

There's a couple of ways in which the ignitors be checked.

First, note that one of the ignitors is used to run the tachometer. If the tach goes wonky when the bike misbehaves, the ignitor running the 1-3 cylinders is a prime suspect, however keep in mind that things other than the ignitor such as loose connections or a bad coil could also cause problems with the tach. On bikes equipped with a fuel pump, the 2-4 cylinder ignitor runs the fuel pump relay. The symptoms here would be the bike suddenly acting like two cylinders dropped out and then a short time later completely dying as the rear cylinders run out of gas, however if the ignitor just goes nuts and supplies intermittent or poorly timed sparks, the fuel pump will continue to operate.

Best thing to try in order to try to narrow down the problem is to swap the connectors to the ignitors and see if the symptoms move to the other pair of cylinders. If the problem moves when swapping the ignitors, you've probably found the problem.

You can also check for dead cylinders by _carefully_ checking to see if the front and rear exhaust pipes are hot or cold. I don't recommend trying to touch them, as the pipes can be extremely hot and will burn you before you can react to the heat. Rather, just try and get near them and feel for radiated heat. If you find cold pipes, try swapping where the ignitors plug in. If swapping the ignitors causes the other bank of cylinders to go dead, you've probably found the problem.

NOTE: In some of these bikes, one ignitor is physically larger than the other and may have a different colored connector, but the two units should still be interchangeable. The larger of the two units has a rev limiter built in but is otherwise functionally equivalent.

If you swap ignitor positions and suddenly everything runs normally, you either corrected a poor connection at the ignitor connector or the disturbing of the wiring has temporarily gotten an intermittent connection within the ignitor to work again. In either case, you probably want to check out the ignitor for solder problems detailed in the second bullet below or problems are likely to reappear in the future.

OK, now you've found a dead ignitor, now what? There's a few possibilities of what the problem is:


* Moisture has gotten into the ignitor and is interfering with proper operation. Some of the bikes are prone to this happening because of poor placement of the ignitors, such as the V45 Magna. Remove the ignitor from the bike, shake it, and see if any moisture comes out of the drain holes. If so, opening the case and drying it out internally may restore normal operation. There is a chance that the moisture has damaged the ignitor (most likely it'd be corrosion damage), but many return to normal operation after drying out. If yours starts working, try and figure out how the moisture is getting in and figure out a way to prevent it in the future. Even if moisture is the problem, you may want to check for the next problem as well, as it's common enough and could be the cause of more problems in the future.

* A solder joint or other mechanical problem has developed in the ignitor. This is actually pretty common. Time and thermal cycling weakens solder joints and they eventually crack and become open or intermittent. The most common problems occur at where the wiring harness connects to the internal PC board. To fix this, you'll have to get the PC board out of the case. This is easier said than done.

http://i339.photobucket.com/albums/n464/geop69/s_ign_rem.jpg

First, pop the end cap off of the ignitor and slide the rubber grommet that the wires go through off the edge of it. The cap is not glued to the case, but is snapped in place. Next you need to slide the PC board out. Sometimes this is relatively easy, sometimes it's a real bitch. The PC boards of the units I have opened were not glued in place, but they had been covered with a sort of waxy material that was used to protect the board's components from the elements. This waxy substance was on the edge of the board as well has over the components and effectively glued it in place. The ones I've pulled apart so far have come loose without damage to the PC board. I use a small jewelers screwdriver to carefully pry on one of the harness wires right at the surface of the PC board and the side of the case. You need to be careful to pry very near the PC board surface and attempt to not put any side loading on the PC board itself or it might crack.

Once the board is out, inspect the solder joints. The pictures to the right are examples of what poor solder joints look like. If you see what appears to be a ring around where the leads are soldered to the boards, this is an indication of cracks that have developed in the solder joint and it is well on its way to failing if it hasn't already. One of the pictures shows the rings, the other shows a large fissure that has developed and the wire is free to pivot around the solder joint. It was a miracle this ignitor was working at all. Use a soldering iron in the ballpark of 20 watts and resolder the joints using rosin core solder. Proper technique is to heat the joint, then add the solder after the joint has melted. You should end up with a nice shiny cone at the joint and if this is done properly, it should only be necessary to heat the joint for a couple of seconds. Extended exposure to the iron could lift the trace or otherwise damage the board. Do not use acid core solder or the acid from the flux will eventually eat away the copper traces on the PC board.

Inspect the other parts of the board as well for any bad solder joints and resolder/repair as necessary.






# The other possibility is that an electrical component has failed. While I consider this the least likely failure scenario, it does happen and at the moment there's not much you can do about it. The active components used in the ignitor are either obsolete or custom and obsolete. The driver transistor (the part on the aluminum heat sink) is the only major component (other than some of the discretes) that looks like it would be possible to find a replacement for. I haven't personally tried these, but the transistor used appears to cross to an NTE2315 or BU806/BU807 type of transistor.



Ignition Troubleshooting Checklist:

All cylinders have problems?

* Check battery
* Check ignition switch
* Check main fuse
* Check kill switch
* Check harness ground connections
* Check both ignitors
* Move on to other possibilites like fuel delivery

Two cylinders have problems that do not move when ignitors swapped

* Check power connections at coil
* Check plug wires
* Check plugs
* Check coils
* Check pulse generators

Two cylinders have problems that do move when ignitors swapped

* Check ignitor connections
* Troubleshoot ignitor

 

I totally agree with the above posted How To. The repair attempt I did included the replacement of the output transistor, type was something like 2SC10xx.
I found the exact same part but got no luck in making the CDi work again. Problem was traced to the CDi's daughter board; took me a lot of time to remove the aluminum cover it had and the material the daughter board was filled with (something like silicone rubber). Then I found the custom components and gave up.